With the development of the economy, the demand for information is increasing rapidly. Optical fiber provides an enormous potential capacity of about 30 THz, and thus fiber communications has become one of the most important technologies for supporting growth of communication services. The OTN standard developed by the International Telecommunication Union—Telecommunication Standardization Sector (ITU-T) lays a foundation for constructing a basic OTN.
In an OTN, the technology for mapping and wrapping client signals to make them suitable for transmission in the OTN is called Digital Wrapping (DW) technology. DW technology involves technical means such as Optical Channel Transport Unit (OTU) mapping, multiplexing structures, time division multiplexing of Optical Channel Data Unit-k (ODUk), and client signal mapping.
Before transmitting client signals, it is necessary to map the client signals to an Optical Channel Payload Unit-j (OPUj), where j represents the supported bit rate and may have the values of 1, 2, or 3 which indicate a bit rate of about 2.5 Gbps, 10 Gbps, and 40 Gbps respectively, and add the overhead of the OPUj into the client signal to constitute an OPUj, and then add the channel overhead of the Optical Channel Data Unit (ODUj) into the OPUj to constitute an ODUj. The OTU overhead and the Forward Error Correction (FEC) overhead are added into the ODUj to constitute an Optical Channel Transport Unit-j (OTUj), and then the OTUj is loaded to a wavelength and sent out.
Time division multiplexing may be performed for the ODUj first so that the client signals can be transmitted through a transport channel with higher rates. Therefore, the G.709 recommendation defines an Optical Channel Payload Unit-k Tributary Slot (OPUk TS) and an Optical Channel Data Tributary Unit j into k (ODTUjk), where k represents the supported bit rate and is greater than j. On the basis of such definition, each byte of the ODUj is mapped to each byte of the ODTUjk in the asynchronous mode, and then the ODTUjk is mapped to the OPUk TS. Finally, an OTUk is constituted for transmitting.
In the step of mapping the client signal to the OPU, in order to transmit client signals of different types, the OTN specifications provide multiple service mapping methods such as mapping of the signals of a Constant Bit Rate (CBR), mapping of the Generic Framing Procedure (GFP) frame, and mapping of the Asynchronous Transfer Mode (ATM) cell flows, which are defined in the G.709. With the growth of data services, new requirements are raised for the full-rate transparent transmission capability of the OTN, and the application of the CBR mapping mode becomes more widespread.
The G.709 living list SP13 puts forward an agnostic CBR mapping method. FIG. 1 shows a frame structure suitable for this CBR mapping. Starting from the 15th column, each OPUk frame includes: a 6-byte Cbyte, where the Cbyte indicates the number of bytes of the mapped client signal; an OPUk payload area composed of (4*3808+1) bytes, for storing client signals; and a 1-byte Payload Structure Identifier (PSI). On the basis of frame structure as shown in FIG. 1, the client signal is mapped to the payload area of the OTN frame of the agnostic CBR service through the existing Σ-algorithm.
In the process of implementing the present invention, the inventor finds that the existing agnostic CBR mapping method uses the fixed frame structure in FIG. 1 to map the client signals. When the rate of the client signal is lower than the nominal value of the OPUk, the positions not stuffed with client signals in the OPUk need to be stuffed with invalid bytes in order to meet the requirements of CBR transmission in the OTN system, thus leading to low bandwidth utilization ratio of the transmission channel. Especially in the case that the client signal rate is low as compared with the nominal value of the OPUk, the OPUk needs to be stuffed with many invalid bytes, thus drastically reducing the bandwidth utilization ratio of the transmission channel. In addition, the definition of the OPUk TS structure in the existing G.709 is limited to the multiplexing from the ODUj to the ODUk, and the existing G.709 defines only 4 OPUk TSs or 16 OPUk TSs as regards the TS allocation. Moreover, the existing G.709 defines only the mapping path of the SDH service as regards the mapping of the CBR service.
With the rapid development of data services, more and more information is transmitted over the Ethernet, Fiber Channel (FC), and Enterprise Systems Connection (ESCON) interface, and such interfaces provide numerous bit rates. For client signals having numerous bit rates, the OTN system defines only the CBR transmission channels and limited CBR mapping methods, and provides no flexible mapping method suitable for CBR transparent transmission of client signals having different bit rates.